ARTS打卡计划第5周-ALGORITHM

168. Excel表列名称

public class Solution168 {
	 public String convertToTitle(int n) {
		String restult = "";
		 while(n>0) {
			 n--;
			 restult =(char)('A'+(n%26))+restult;
			 n=n/26;
			 
		 }
		return restult;
	        
	    }
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Solution168 s = new Solution168();
		System.err.println(s.convertToTitle(26));
		System.err.println(s.convertToTitle(52));
		System.err.println(s.convertToTitle(27));
		System.err.println(s.convertToTitle(26));
		System.err.println(s.convertToTitle(28));
		System.err.println(s.convertToTitle(701));
	}

}

　　

204. 计数质数

public class Solution204 {
	public int countPrimes(int n) {
		int result = 0;
		for (int i = 1; i < n; i++) {
			if (isPrime(i)) {
				result++;
			}
		}
		return result;
	}

	/**
	 * AKS算法计算一个数是否是素数
	 * https://stackoverflow.com/questions/1801391/what-is-the-best-algorithm-for-checking-if-a-number-is-prime
	 * @param n
	 * @return
	 */
	public boolean isPrime(int n) {
		if (n == 1) {
			return false;
		}
		if (n == 2 || n == 3) {
			return true;
		}
		if (n % 2 == 0 || n % 3 == 0) {
			return false;
		}
		int i = 5, w = 2;
		while (i * i <= n) {
			if (n % i == 0) {
				return false;
			}
			i += w;
			w = 6 - w;
		}

		return true;

	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Solution204 s = new Solution204();
		System.out.println(s.countPrimes(10));
	}

}

　

179. 最大数

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

public class Solution179 {
    public String largestNumber(int[] nums) {
    	List<String> lists =new ArrayList<String>(nums.length);
    	for(int num:nums) {
    		lists.add(num+"");
    	}
    	lists.sort(new Comparator<String>() {

			@Override
			public int compare(String o1, String o2) {
				// TODO Auto-generated method stub
				return (o2+o1).compareTo(o1+o2);
			}  		
		});
    	if(lists.get(0).equals("0")) {
    		return "0";
    	}
    	StringBuffer sb = new StringBuffer();
    	for(String numStr:lists) {   		
    		sb.append(numStr);
    	} 	
    	return sb.toString();
    	  
    }
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Solution179 s = new Solution179();
		int[] nums = new int[] {3,30,34,5,9};
		System.out.println(s.largestNumber(nums)); 
	}

}

　　这道题先思考清楚本质是一个排序问题，坑的地方在于0的处理上。